Here are the links to the 6 versions of these HP memory Quickspecs. Only version 1 and 2 have anything related to the Z400/Z600/Z800. I tried HP ECC registered sticks that were certified by HP for the Z600 v2 in a few Z400 v2 workstations and they did not work. Here are those links, and none exist after version 6:
https://www8.hp.com/h20195/v2/getpdf.aspx/c04164501.pdf?ver=1
https://www8.hp.com/h20195/v2/getpdf.aspx/c04164501.pdf?ver=2
https://www8.hp.com/h20195/v2/getpdf.aspx/c04164501.pdf?ver=3
https://www8.hp.com/h20195/v2/getpdf.aspx/c04164501.pdf?ver=4
https://www8.hp.com/h20195/v2/getpdf.aspx/c04164501.pdf?ver=5
https://www8.hp.com/h20195/v2/getpdf.aspx/c04164501.pdf?ver=6